Method And System For Amplifying A Signal Using A Transformer Matched Transistor

ABSTRACT

A power amplifier includes a transistor, a transmission line transformer, and a capacitor. The transistor is operable to receive a signal and to generate an amplified signal. The transistor has a source, a drain, and a gate. The gate has a first impedance and is operable to receive the signal to be amplified. The transmission line transformer has a first, second, third, and fourth port, the first port being coupled to the gate of the transistor and the third port, and the fourth port being coupled to a source device having a second impedance. The capacitor has a first end and a second end. The first end of the capacitor is coupled to the second port of the transmission line transformer and the second end is coupled to a ground.

TECHNICAL FIELD

This disclosure relates generally to the field of power amplifiers andmore specifically to a method and system for amplifying a signal using atransformer matched transistor.

BACKGROUND

A typical power amplifier includes one or more transistor stages. Eachtransistor stage supplies excitation to an input signal from a signalsource in order to amplify the input signal and provide the amplifiedsignal to a next transistor stage or a receiver.

In order to efficiently transfer the input signal from the signal sourceto the output transistor stage of a power amplifier, the input impedanceof the output transistor stage needs to closely match the impedance ofthe signal source. The input impedance to each transistor stage of apower amplifier, however, is typically very small and difficult tomatch.

SUMMARY OF THE DISCLOSURE

According to one embodiment of the present disclosure, a power amplifierincludes a transistor, a transmission line transformer, and a capacitor.The transistor is operable to receive a signal and to generate anamplified signal. The transistor has a source, a drain, and a gate. Thegate has a first impedance and is operable to receive the signal to beamplified. The transmission line transformer has a first, second, third,and fourth port, the first port being coupled to the gate of thetransistor and the third port, and the fourth port being coupled to asource device having a second impedance. The transmission linetransformer is operable to efficiently transport a signal from thesource device to the transistor by matching the first impedance andsecond impedance. The capacitor has a first end and a second end. Thefirst end of the capacitor is coupled to the second port of thetransmission line transformer and the second end is coupled to a ground.

Certain embodiments of the disclosure may provide one or more technicaladvantages. A technical advantage of one embodiment may be that an inputsignal from a signal source may be more efficiently transferred to anoutput transistor of a power amplifier. The improved transfer of theinput signal may improve the bandwidth and efficiency of the poweramplifier. Other advantages may include requiring standard foundryprocesses to implement the transmission line transformer. This resultsin more economical circuits and a reduction in overall system costs.Additionally, the use of a capacitor connecting the transmission linetransformer to ground allows a transistor gate DC bias to be suppliedalong with the input signal. The DC bias may allow more efficienttransistor operation.

Certain embodiments of the disclosure may include none, some, or all ofthe above technical advantages. One or more other technical advantagesmay be readily apparent to one skilled in the art from the figures,descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a circuit diagram of one embodiment of a power amplifier thatmay be used to amplify a signal;

FIG. 2 is a circuit diagram of an example power amplifier of theembodiment of FIG. 1;

FIG. 3 is a flow chart illustrating a signal amplification method inaccordance with a particular embodiment of this disclosure; and

FIGS. 4A and 4B illustrate a transmission line transformer in accordancewith a particular embodiment of this disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure and its advantages are bestunderstood by referring to FIGS. 1 through 4 of the drawings, likenumerals being used for like and corresponding parts of the variousdrawings.

FIG. 1 is a diagram of a power amplifier system 100 that may be used toamplify a signal. Power amplifier system 100 includes a signal source110, an impedance match device 120, a power amplifier 130, and a signalreceiver 140. Signal source 110 is coupled to impedance match device 120and provides an input signal 150 to be amplified. Impedance match device120 is coupled to and provides a matched signal 160 to power amplifier130. Power amplifier 130 is coupled to and provides an amplified signal170 to signal receiver 140.

In operation, signal source 110 provides input signal 150 to impedancematch device 120. Signal source 110 may be any device that supplies asignal such as a signal generator, a cable, another power amplifier 130,and the like. In some embodiments, signal source 110 may also supply aDC bias to input signal 150 in order to control the operation of poweramplifier 130. Input signal 150 may be any electrical signal including,but not limited to, signals utilized in radars, communication systems,electronic warfare systems, and the like, and may include a DC offsetcomponent. Impedance match device 120 receives input signal 150 andoutputs matched signal 160. Matched signal 160 may be identical to inputsignal 150 except for any loss associated with the transfer of inputsignal 150 from signal source 110 through impedance match device 120.Power amplifier 130 receives matched signal 160, amplifies matchedsignal 160, and outputs amplified signal 170 to signal receiver 140.

Impedance match device 120 attempts to match the input impedance ofpower amplifier 130 as closely as possible to the impedance of signalsource 110 in order to efficiently transfer input signal 150. Forexample, if signal source 110 is a typical transmission cable found in acommunications system, it may have an impedance of approximately 50ohms. Additionally, if power amplifier 130 is a typical last stage of ahigh-power amplifier, it may have an input impedance of approximately 1ohm. Impedance match device 120 is then utilized to match the 50 ohmimpedance of the cable to the 1 ohm input impedance of the high-poweramplifier so that matched signal 160 is as identical to input signal 150as possible.

Typical power amplifiers such as power amplifier system 100 employ ashunt capacitor immediately on the gate of an output transistor of thepower amplifier in order to match the higher impedance of the signalsource with the lower impedance of the power amplifier. While shuntcapacitors in systems such as these are effective at matching theimpedances, they typically cause the power amplifier to have a narrowbandwidth.

The teachings of the disclosure recognize that it would be desirable tobe able to transform the lower impedance of a power amplifier input intoa higher impedance while increasing the bandwidth of the power amplifierover systems utilizing typical shunt capacitors. FIGS. 2 and 3 belowillustrate a system and method of addressing problems associated withtypical shunt capacitors and power amplifiers.

FIG. 2 is a circuit diagram illustrating in more detail a portion ofpower amplifier system 100 according to the teachings of the disclosure.FIG. 2 includes coupled lines connected as a transmission linetransformer 210, a capacitor 220, a transistor 230, and a ground 240.

Transmission line transformer 210 is well known in the art and includesa port 212, a port 214, a port 216, and a port 218. Capacitor 220includes a first end 222 and a second end 224. Capacitor 220 may be anysize of capacitor that is sufficient to provide a low RF impedance, andgenerally depends on the frequency of input signal 150. Transistor 230may include a source 136, a drain 134, and a gate 132, and may be afield effect transistor. Transistor 230 may be, for example, an outputtransistor of an amplifier circuit. Port 212 is coupled to signal source110. Port 216 is coupled to port 214, and is further coupled to gate 132of power amplifier 130. Port 218 is coupled to first end 222 ofcapacitor 220, and second end 224 of capacitor 220 is coupled to ground240. Drain 134 of transistor 230 is coupled to signal receiver 140, andsource 136 of transistor 230 is coupled to ground 240.

In operation, transmission line transformer 210 may be utilized asimpedance match device 120 as described above in reference to FIG. 1 inorder to efficiently transmit input signal 150 from signal source 110 togate 132 of transistor 230. Transmission line transformer 210 firstreceives input signal 150 via port 212 from signal source 110. Inputsignal 150 may be any electrical signal as described above and mayinclude a DC offset component. Signal source 110 has an output impedancethat may be approximately 50 ohms as described above. It should benoted, however, that signal source 110 may have any impedance andtransmission line transformer 210 is not limited to receiving signalsfrom a source having a 50 ohm impedance. Once transmission linetransformer 210 receives input signal 150, transmission line transformer210 transports input signal 150 and outputs matched signal 160 to gate132 of transistor 230 via port 216. Transistor 230 then amplifiesmatched signal 160 and outputs amplified signal 170 to signal receiver140 via drain 134.

Transistor 230 may have an input impedance of approximately 1 ohm atgate 132. Transmission line transformer 210, in combination withcapacitor 220 as shown in FIG. 2, matches the input impedance oftransistor 230 at gate 132 to the output impedance of signal source 110at port 212. As a result, matched signal 160 arrives at gate 132 withminimal signal loss. In addition, the bandwidth of transistor 230 isincreased over typical power amplifiers that utilize a shunt capacitordirectly on gate 132 of transistor 230.

Most transistors in power amplifier applications do not operate wellwithout a DC gate bias. Additionally, most transmission linetransformers couple port 218 directly to ground and are therefore unableto pass a DC bias through to a coupled transistor. However, poweramplifier system 100 overcomes these problems by coupling capacitor 220to port 218 of transmission line transformer 210. As a result, a DC gatebias may be passed through transmission line transformer 210 via inputsignal 150 and be applied to gate 132 of transistor 230. This may allowfor more efficient operation of transistor 230.

In some embodiments, source device 110 may be another transmission linetransformer 210. In these embodiments, back-to-back transmission linetransformers 210 are utilized to achieve a desired bandwidth notavailable by utilizing a single transmission line transformer 210.

FIG. 3 illustrates an example signal amplification method 300 that maybe utilized to efficiently and effectively amplify a signal accordingteachings of the disclosure. Signal amplification method 300 begins instep 310 where an input signal to be amplified is received at an inputnode from a signal source. The input signal may be, for example, inputsignal 150 as described above. In step 320, the input node is coupled toa transistor with a transmission line transformer (“TLT”). Thetransistor may be transistor 230 as described above, and the TLT may betransmission line transformer 210 as described above. The TLT is coupleddirectly to the gate of the transistor and is utilized to match theimpedance the gate of the transistor with the signal source.

In step 330, one port of the TLT is coupled to a ground via a capacitor.The capacitor maintains the electrical connection of the TLT whileisolating the gate DC bias from ground and delivering a DC bias throughthe TLT to the transistor. In step 340, the input signal is received atthe gate of the transistor via the TLT. Finally, the input signal isamplified by the transistor in step 350.

FIGS. 4A and 4B illustrate an example transmission line transformer 410that may be implemented as transmission line transformer 210 above. Insome embodiments, transmission line transformer 410 is implemented in amicrowave monolithic integrated circuit (“MMIC”) amplifier to match theimpedance of an output transistor. Unlike typical MMIC transmission linetransformers which are formed by utilizing expensive additionaldielectric layers, however, transmission line transformer 410 is formedusing standard foundry processes. As a result, transmission linetransformer 410 is more economical.

Transmission line transformer 410 includes a top line 420 formed on atop circuit board layer 430, a bottom line 440 formed on a bottomcircuit board layer 450, and a dielectric 460 that is between top layer430 and bottom layer 450. Together, top line 420, bottom line 440, anddielectric 460 form a broadside microstrip coupler and are created usingstandard capacitor elements. Top line 420 is coupled to port 212 on oneend and port 216 on the other end as shown in FIG. 4. Likewise, bottomline 440 is coupled to port 214 on one end and port 218 on the other endas shown in FIG. 4. Port 216 is coupled to port 214 and gate 132 oftransistor 230, and port 218 is coupled to capacitor 220.

In operation, an input signal such as input signal 150 above arrives atport 212 from signal source 110. Once transmission line transformer 410receives input signal 150, transmission line transformer 410 transportsinput signal 150 via top line 420 and bottom line 440 to port 216.Transistor 230 then amplifies matched signal 160 and outputs amplifiedsignal 170 to signal receiver 140 via source 134.

Although the embodiments in the disclosure have been described indetail, numerous changes, substitutions, variations, alterations, andmodifications may be ascertained by those skilled in the art. Forexample, FIGS. 4A and 4B depict top line 420 on top circuit board layer430 and bottom line 440 on bottom circuit board layer 450. Otherembodiments, however, may implement top line 420 on bottom circuit boardlayer 450 and bottom line 440 on top circuit board layer 430.Additionally or alternatively, other embodiments may utilize a bipolarjunction transistor having a base, a collector, and an emitter ratherthan transistor 130 having gate 132, drain 134, and source 136. It isintended that the present disclosure encompass all such changes,substitutions, variations, alterations and modifications as fallingwithin the spirit and scope of the appended claims.

1. A circuit for amplifying a signal, comprising: a transistor operableto receive a signal to be amplified and to generate an amplified signal,the transistor having a source, a drain, and a gate, the gate having afirst impedance and operable to receive the signal to be amplified; atransmission line transformer having a first, second, third, and fourthport, the first port coupled to the gate of the transistor and the thirdport, and the fourth port coupled to a source device having a secondimpedance, the transmission line transformer operable to efficientlytransport a signal from the source device to the gate of the transistorby matching the first impedance and the second impedance; and acapacitor having a first end and a second end, the first end coupled tothe second port of the transmission line transformer and the second endcoupled to a ground.
 2. The circuit of claim 1, wherein the transmissionline transformer comprises a broadside microstrip coupler.
 3. Thecircuit of claim 2, wherein the broadside microstrip coupler comprisescapacitor elements, the capacitor elements further comprising a top lineand a bottom line, the top line residing on a top circuit board layerand the bottom line residing on a bottom circuit board layer, the topcircuit board layer being separated from the bottom circuit board layerby a dielectric.
 4. The circuit of claim 1, wherein the source devicecomprises a second transmission line transformer.
 5. The circuit ofclaim 1, wherein the transistor, the transmission line transformer, andthe capacitor comprise devices in a microwave monolithic integratedcircuit.
 6. The circuit of claim 1, wherein the signal comprises anelectrical signal having a DC bias and the transmission line transformeroperable to pass the DC bias from the source device to the gate of thetransistor.
 7. A method for coupling a signal to a power amplifiercomprising: receiving a signal to be amplified from an output node of asource device; impedance matching the output node of the source deviceto an input gate of a transistor by a transmission line transformer; andtransmitting the signal from the output node of the source device to theinput gate of a transistor with the transmission line transformer. 8.The method of claim 7 further comprising: receiving the signal at thegate of the transistor; amplifying the signal with the transistor tocreate an amplified signal; transmitting the amplified signal to areceiver.
 9. The method of claim 7, wherein the transmission linetransformer comprises a broadside microstrip coupler.
 10. The method ofclaim 9, wherein the broadside microstrip coupler comprises capacitorelements, the capacitor elements further comprising a top line and abottom line, the top line residing on a top circuit board layer and thebottom line residing on a bottom circuit board layer, the top circuitboard layer being separated from the bottom circuit board layer by adielectric.
 11. The method of claim 7, wherein the source devicecomprises a second transmission line transformer.
 12. The method ofclaim 7, wherein the transistor, and the transmission line transformercomprise devices in a microwave monolithic integrated circuit.
 13. Themethod of claim 8, wherein the receiver comprises a second transistor.14. The method of claim 7 further comprising transmitting a DC bias fromthe output node of the source device to the input gate of a transistorwith the transmission line transformer.
 15. A microwave monolithicintegrated circuit comprising: a transistor having a gate operable toreceive a signal to be amplified, the gate having a first impedance; atransmission line transformer having a first, second, third, and fourthport, the first port coupled to the gate of the transistor and the thirdport, and the fourth port coupled to a source device having a secondimpedance; and a capacitor having a first end and a second end, thefirst end coupled to the second port of the transmission linetransformer and the second end coupled to a ground; wherein thetransmission line transformer comprises a broadside microstrip coupler.16. The microwave monolithic integrated circuit of claim 15, wherein thesource device comprises a second transmission line transformer.
 17. Themicrowave monolithic integrated circuit of claim 15, wherein thebroadside microstrip coupler comprises capacitor elements, the capacitorelements further comprising a top line and a bottom line, the top lineresiding on a top circuit board layer and the bottom line residing on abottom circuit board layer, the top circuit board layer being separatedfrom the bottom circuit board layer by a dielectric.
 18. The microwavemonolithic integrated circuit of claim 15, wherein the transistor is anoutput transistor of an amplifier.
 19. The microwave monolithicintegrated circuit of claim 15, wherein the transmission linetransformer is operable to match the first impedance and the secondimpedance.
 20. The microwave monolithic integrated circuit of claim 15,wherein the signal to be amplified comprises an electrical signal havinga DC bias and the transmission line transformer operable to pass the DCbias from the source device to the gate of the transistor.